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Here are a bunch of sites that talk about the Bosch D-jet setup (external links):

Some of the early to mid 70's cars used the same "computer" as in the coupe (eg. Volvo 164) and may be easier to find.

-----Original Message-----
From: Mike and Marci Clarkenfields
Sent: Sunday, December 23, 2001 9:04 AM
To: gopostal@seniorsix.org
Subject: D-Jet Tuning- Long!

I'll start off by stating that most of my experience with D-Jet is with Volvos and Volkswagens. So there may be some minor differences with the BMW setup. All of this assumes that you are a fairly competent mechanic. If you cannot go to your engine and point out the various FI components, you should probably let a shop handle this chore.

D-Jet tuning techniques:
New air filter!
Check all vacuum hoses for leaks.
Use contact cleaner and clean all FI electrical connections- particularly the water temperature sensor.
Clean the ground wires.
Now we can start.

1. Pull the spark plugs

2. Adjust the valves

3. Set timing to #1 cyl TDC, pull the distributor cap, mark the rotor position and distributor position.

4. Pull the distributor.

5. Pull the FI trigger points in the distributor and clean them with contact cleaner, check condition of the rubbing blocks, lube the rubbing blocks with a very light dab of grease, and reinstall in distributor. Hook up a multi-meter and give the distributor a twirl and verify that both sets of points are doing their thing (an old FI plug and piece of harness will simplify this procedure tremendously)

6. Clean and set the ignition points

7. Reinstall the distributor with the rotor pointing to the mark you made earlier.

8. Install new spark plugs (check gap, use anti-seize on threads) and start the car and set the timing, shut it back off.

9. Pull the air-bleed screw out of the manifold and clean it with carb cleaner. Spray the air-bleed passages as well and clean the throttle plate. Reinstall the air-bleed screw till it bottoms, then back it out 2 1/2 turns.

10. Pull the cover on the throttle position switch (TPS). Use an ink eraser and clean the printed circuit board- pay particular attention to where the wipers sweep across the board. Spray the board, contact points, and connector pins with contact cleaner, let dry.

11. Check your accelerator function by turning the car on (don't start it) and running the throttle ONCE very slowly through the complete length of travel. As the contact arms of the TPS sweep across the board, you should hear the injectors fire- click, click, click.

12. Turn the car off. With the TPS cover still off, run the throttle through a couple of cycles and watch how the contact points (switches) work. At the idle position, one set is closed, and one is open. Off idle, the other set is open and the first set is closed. If this is not happening, loosen the two screws that let the TPS rotate relative to the throttle shaft and rotate it until the switches operate as described. Tighten the screws down. These switches control how the brain box adjusts the air/fuel mixture. When the TPS is in the idle position, it allows the mixture to be adjusted with the knob on the brain box, when it comes off of the idle position, the mixture is controlled by the manifold air pressure (MAP) sensor. Start the car and listen to it run (at idle). Mark the position of the knob on the brain box FIRST. Turn the knob one click at a time (right to richen, I think). Stop, listen for at LEAST a minute to see how the car runs. Set the mixture for the highest rpm, and back off one click. This should put the mixture in the best emissions range- the highest rpm setting tends to be a tad rich, but can be used if the car doesn't get a tailpipe test.

At this point the car should be running (idling) fairly well. Generally speaking, the air temp sensor and the water temp sensor are fairly bullet-proof, but if the idle is still rough after the above, the these should be checked. If you are having problems with idle speed, check the warm-up air regulator. This should suck a lot of air when it is cold, and go down to almost no air flow when the car is warm. This is a common failure point on D-Jets cars. If the air regulator fails in the closed position, the car will be hard to start and tend to die when cold, if it fails in the open position, you won't be able to get the idle low enough.

If the idle is good, but the car is still running crappy:

1. Go to the manifold air pressure (MAP) sensor. Look at the side where there is a blob of epoxy in the center of the MAP sensor. Carefully drill, chip or file the epoxy off. This should reveal a screw or there may just be a hole in the side of the MAP sensor. Inside this hole is the mixture adjustment screw- a small blade screwdriver will fit in it.

2. Start the car. Loosen the set screws on the TPS, and rotate it until the off-idle points are open. This means that the idle mixture is now being controlled by the MAP sensor. Use a screwdriver and find the slot position of the adjustment screw. MARK your starting position!

3. Slowly turn the adjustment screw- NO MORE THAN 5 DEGREES AT A TIME. Right to richen, left to lean. Wait for a minute for a change and the car to settle down. If the car responds, set for the highest rpm- dial it in, but slowly. Once the highest idle speed is set with the MAP sensor, go back to the TPS and rotate it back to the proper position. Usually the idle will drop slightly.

4. Take the car on a test drive. If the mixture is set properly, it will run fine. If it is too lean, it will surge when it is cruising in a "light load" steady condition. I will usually tweak it leaner until it surges, then richen it until the surging just stops.

If the car does not respond to tweaks in the MAP sensor, then the sensor is most likely bad. When these cars were new, the MAP sensor rarely failed, at the age of these cars now, chances are high that it has failed.

Good luck!
Mike
70 2500- Wallis
72 Bavaria- Baby
72 R75/5